As is well known and understood, one of the major concerns of designers of antenna systems for communications links is the elimination or reduction of external interference sources such as jamming, self-interference, atmospheric noise, and man made noise. Also as is well known, most arrangements which attempt to resolve these problems of external interference do so in a relatively complex manner, often utilizing very large directional antennas and/or with antennas having hundreds or more elements. This problem of external interference is particularly prevalent in the area of mobile communications systems where omnidirectional antennas are employed because of the large number of users operating in the same frequency band and because of multipath.
A relatively recent approach for eliminating interference in a communications system utilizing a plurality of transmission links is shown and described in U.S. Pat. No. 4,275,397, entitled, "Interference Cancelling Random Access Discrete Address Multiple Access System", which issued to Frank S. Gutleber, the present inventor, on June 23, 1981. The system disclosed therein utilizes orthogonal multiplexing in conjunction with the receiver antenna configuration comprised of an omnidirectional antenna and a notch antenna at the receiving end of the transmission link to cancel interference arriving from all directions except over the narrow beamwidth or null formed by the notch antenna.
As is well known, a notch antenna comprises an antenna having a pattern which contains uniform reception in all directions except for a relatively small angular beamwidth where there is formed a null having a relatively steep slope. General design procedures for providing this type of an antenna pattern have heretofore been described in U.S. Pat. No. 3,130,410, entitled, "Space Coded Linear Array Antenna", F. S. Gutleber, Apr. 21, 1964, and U.S. Pat. No. 3,605,106, entitled, "Slot Fitting Of Code Linear Array Antenna", Frank S. Gutleber, Sept. 14, 1971, and the above referenced related patent application Ser. No. 533,089 (CECOM D-2891), entitled, "A Linear Array Antenna Employing The Summation Of Subarrays". Such patterns are made up of product and/or sums of (sin mx/sin x) functions, and can be achieved by controlling both the amplitudes and spacings of array antenna elements. As a result, the slope of a null in an antenna beam pattern can be made steep, either by providing one or more (sin mx/sin x).sup.y terms or by appropriate amplitude and phase controls when summing several (sin mx/sin x) functions using subarrays.
An alternative embodiment of a notch antenna can be achieved by utilizing a pair of antenna patterns having a somewhat wide null beamwidth, but where one is angularly displaced by a small amount with respect to the other. The two beams can be positioned so that the desired signal is near the edge of one receiving beam while being nulled out of the second receiving beam.
Accordingly, it is an object of the present invention to eliminate external interference at one end of a transmission link.
Another object of the invention is to provide an antenna whose beamwidth pattern contains uniform reception from all directions except a small angular sector for eliminating external interference at one end of a transmission link.
Still a further object of the invention is to provide a steep notch antenna pattern for application in tactical communications systems employing interference cancellers.